In recent years, the information recording technique, particularly the magnetic recording technique, has remarkably advanced following the development of information technology. As a magnetic recording medium, being one of magnetic recording media, to be mounted in an HDD (hard disk drive) or the like, there is a magnetic disk. The magnetic disk is formed by coating a film of NiP (nickel phosphorus) or the like on a metal substrate made of an aluminum-magnesium alloy or stacking an underlayer, a magnetic layer, a protective layer, and a lubricating layer in this order on a substrate such as a glass substrate or a ceramic substrate. Aluminum substrates have conventionally been widely used as magnetic disk substrates. However, following the reduction in size and thickness and the increase in recording density of magnetic disks, glass substrates excellent in substrate surface flatness and substrate strength as compared with the aluminum substrates have been gradually replacing them.
The glass substrates with high rigidity are also advantageous in terms that the improvement in impact resistance is also required for mounting large-capacity magnetic recording media in mobile devices and automobiles. The size of substrates tends to be reduced for installation in mobile devices. Accordingly, starting from conventional 3.5-inch substrates, there have been required 2.5-inch substrates, 1.8-inch substrates, 1-inch substrates, and smaller substrates. As the size of the substrates decreases, the allowable dimensional error also decreases and thus more accurate shape processing is required.
Further, following the increase in density of the magnetic recording technique, magnetic heads have also shifted from thin film heads to magnetoresistive heads (MR heads) and to giant magnetoresistive heads (GMR heads), wherein the flying height of a magnetic head from a substrate has decreased to even 10 nm or less. However, when the magnetic head flies over a magnetic disk with such an extremely low flying height, there is a problem that a fly stiction failure tends to occur. The fly stiction failure is a failure in which a magnetic head flying over a magnetic disk causes abnormality in flying posture or flying height, which causes irregular reproduction output changes. If this fly stiction failure occurs, there may occur a head crash failure in which the flying magnetic head is brought into contact with the magnetic disk. Therefore, the glass substrate surfaces have been required to have high-level flatness and smoothness.
Further, for effectively using the area of the surface of the glass substrate, the load/unload type (Load Unload) has started to be employed in place of the conventional CSS type (Contact Start Stop). The CSS type is a type in which a magnetic head is brought into contact with a substrate surface at the time of disk stoppage, and thus it is necessary to provide a CSS region (region for contact sliding with a magnetic head) on the substrate surface. In contrast, the load/unload type is a type in which a magnetic head is retreated to the outside of a glass substrate at the time of disk stoppage, and thus there is an advantage in that a CSS region can also be used as a recording surface. Further, during stoppage of a magnetic disk device, even if a strong impact is applied, since the magnetic head is retreated, it is possible to minimize damage to a magnetic disk. For a portable small-sized hard disk, a combination of a start reproduction system of the load/unload type and a magnetic disk using a glass substrate is selected in terms of ensuring the information recording capacity and improving the impact resistance.
In the load/unload type, since a magnetic head passes through an end portion of a glass substrate, the shape at an outer peripheral portion of the glass substrate particularly arises as a problem. If there is disturbance in shape (rising or lowering) at the outer peripheral portion of the glass substrate, the flying posture of the magnetic head is disturbed so that a contact tends to occur when the magnetic head comes in from the outside of the glass substrate or goes out, and thus there is a possibility of the occurrence of crash failure. Therefore, high flatness is required particularly at the disk outer peripheral portion.
Not only an increase in density but also an increase in speed is required for magnetic disks. Conventionally, a magnetic disk device mounted with a glass substrate has used a relatively low rotational speed of 4200 rpm or the like. In recent years, however, a rotational speed of, for example, 7200 rpm or more has started to be used. Further, in near future, a rotational speed of 10000 rmp or more is expected to be used. With such high-speed rotation, the linear velocity particularly near the outer periphery of a magnetic disk increases. For example, in a magnetic disk at a rotational speed of 4200 rpm, the linear velocity at a position of radius 32.5 mm from the substrate center is 14.3 m/sec, while, the linear velocity becomes 18.4 m/sec at 5400 rpm and the linear velocity becomes 24.5 m/sec at 7200 rpm. The above-mentioned fly stiction failure and head crash failure particularly tend to occur at the disk outer peripheral portion where the linear velocity becomes high as described above. Therefore, also in this viewpoint, high flatness is required particularly at the outer peripheral portion.
In recent years, the contact-sliding type recording medium (contact-recording type recording medium) has been re-evaluated. The contact-sliding type recording medium is a recording type in which a recording head reads and writes in a state where it is in sliding contact with a magnetic disk. Although the contact-sliding type recording medium itself is the recording type that has been present for a long time, since the recording density can be increased as the distance between a recording head and a magnetic disk is reduced, it is again considered to be the recording type that will be developed in future. As the flying height of a recording head decreases, there is a case where the recording head contacts a magnetic disk. That is, as a result of reducing the flying height of the recording head, there is a case where, partially, the recording head makes a sliding contact with the magnetic disk. However, if it makes the sliding contact, wear of the recording head becomes a big problem. Further, there is also a problem that if the recording head jumps, there is a possibility that the signal quality degrades or the recording head is damaged due to impact upon jumping or landing. These are all largely attributable to unevenness of the surface of the magnetic disk and, as the rotational speed (i.e. the linear velocity) of the magnetic disk increases, the influence increases. Therefore, also in this viewpoint, high flatness is required particularly at the outer peripheral portion.
On the other hand, as described in Patent Document 1 (JP-A-2005-141852), there has conventionally been a problem that when the main surface of a substrate is polished, the flatness of its outer peripheral portion becomes insufficient. That is, a glass substrate is polished by pressing the front and back main surfaces thereof between polishing pads and relatively moving the glass substrate and the polishing pads while supplying a slurry containing abrasives. In this event, rising (the outer peripheral portion of the main surface projects as compared with the other portion of the main surface) called ski jump occurs at the outer peripheral portion of the main surface or lowering (the outer peripheral portion of the main surface falls in a state of being shaved relatively greater than the other portion of the main surface) called roll-off occurs at the outer peripheral portion of the main surface. Either one of the ski jump and the roll-off may occur or both may occur.    Patent Document 1: JP-A-2005-141852